Nitrogen oxides (NO.sub.x) meaning nitrogen monoxide, nitrogen dioxide and nitrous oxide are environmental contaminant materials like carbon oxides and sulfur oxides. Nitrous oxide is not very toxic, but it is one of major contaminants causing global warming like carbon dioxide. Nitrogen monoxide is a major component of nitrogen oxides in exhaust gas, and can be easily converted into nitrogen dioxide even at room temperature when it is discharged into air. Both nitrogen monoxide and nitrogen dioxide are carcinogenic materials. They cause serious air pollution and acid rain like sulfur oxides. The discharge of nitrogen oxides are caused mostly by reaction of nitrogen and oxygen in air during high temperature combustion and combustion of nitrogen compound contained in fuel. There is, therefore, a need to provide a technique of removing nitrogen oxides by treating exhaust gas in addition to controlling the generation of nitrogen oxides through combustion control.
The technique of removing nitrogen oxides is largely divided into two groups depending on their use of catalyst. When a catalyst is used, the technique is divided into a method using reducing agent and a method of decomposing directly on a catalyst without using reducing agent. A method which directly decompose nitrogen oxides in exhaust gas on catalyst into nitrogen and oxygen is referred since nitrogen oxides are unstable thermodynamically as compared to nitrogen and oxygen. However, a suitable catalyst for the method is not found since the method has problems in that it requires high reaction temperature and the activity of the catalyst decreases easily.
The technique of selective catalytic reduction of nitrogen oxides has come to employed since the treatment with ammonia as a reducing agent was practically used and the application of the technique has rapidly increased. Taking into account the fact that the discharge of various environmental contaminating exhaust gas will be severely regulated in advanced nations, it is likely hay the demand of the technique will increase largely. However, the conventional reduction method using ammonia as a selective reducing agent has some problems in that in addition to the problems of difficulty of transport and charge and high equipment maintenance cost due to the corrosion property of ammonia, unreacted ammonia is drawn off into the air and thus cause secondary air pollution. Owing to those drawbacks, recently the research has been devoted to substitution of carbon containing compounds, such as hydrocarbon, methanol or ethanol for a reducing agent.
In order to develop reduction catalysts which directly decompose nitrogen monoxide or in available for selective contact reducing agent, many researches have been investigated concerning metal or metal oxide catalyst of platinum group since 1970. When air to fuel ratio of automobile is stoichiometric, three-way catalyst of platinum-palladium-rhodium is able to convert carbon monoxide, hydrocarbon and nitrogen oxides which are contaminants of exhaust gas to innoxious gas by more than 90%. However, in the case that exhaust gas is dilute under excessive oxygen as in lean burn gasoline engine or diesel engine, the three-way catalyst can not act as reduction catalyst for nitrogen oxides but only as oxidation catalyst. For this reason, copper ion exchanged Cu--ZSM-5 zeolite and various metal oxides catalyst have been developed as catalysts for removing nitrogen oxides for substituting for three-way catalyst of platinum group. According to the reports made until now, copper ion exchanged Cu--ZSM-5 zeolite are said to be the most effective catalyst for converting nitrogen oxides into nitrogen and oxygen.
Japanese Patent No. 03 86 213 according to Hamada et al., suggested the use of various zeolites including modernite, ferrierite, L type and ZSM-5 as catalysts for converting nitrogen oxides into innoxious gas under excessive oxygen. The catalyst shows very high activity on direct decomposition of nitrogen oxides. Iwamoto et al. suggested Cu--ZSM-5 zeolite which is exchanged by copper ion by more than 100% in over pH 6 in Japanese Patent 03 101 837 had excellent effect on converting nitrogen oxides. Many studies have confirmed the possibility of using Cu--ZSM-5 zeolite as direct decomposition catalyst for nitrogen oxides. However, it is sensitive to sulfur oxides and steam, and thus easily lose activity. The activity can be reduced considerably under excessive oxygen and at high space velocity as well. To solve such problems relating to Cu--ZSM-5 zeolite, Iwamoto and Hamada (Shokubai, 32 430 (1990), Appl. Catal., 64, L1 (1990)), and Held et al., (SAE Paper 900496 (1990)) proposed selective catalytic reduction of nitrogen oxides by utilizing hydrocarbon contained in exhaust gas.
Zeolite catalyst systems exchanged by transition metal ion other than copper ion are Co--ZSM-5 disclosed in Appl. Catal. B. 1, L31 (1992) according to Arm et al. (USA) and Ga--ZSM-5 in Japanese Patent 05 212 288. These catalysts showed very high reduction activity of nitrogen oxides due to oxidation of hydrocarbon even in the presence of excessive oxygen in 10% or so and in methane which is nonselective reducing agent. Yokoyama et al. also suggested several kinds of catalyst systems including Ce--ZSM-5 overcome the drawbacks of Cu--ZSM-5 catalyst (Shokubai, 35, 122 (1993).
As a compound having direct decomposition activity of nitrogen oxides, there are supported noble metal catalyst (typically, platinum/alumina), transition metal oxides (typically, cobalt oxides CO.sub.3 O.sub.4), mixed metal oxides (typically, perovskite oxides) and transition metal-exchanged ZSM-5 zeolites (typically, Cu--ZSM-5). Among them, metal oxides or complex thereof are known to have high decomposition activity of nitrogen oxides at high temperature, but to have drastically decreased decomposition activity of NO.sub.x in the presence of excessive oxygen. There are little reports on selective catalytic reduction of mixed metal oxides using hydrocarbon as a reducing agent in the presence of excessive oxygen. Shin et al. reported decomposition activity of SrFeO.sub.3-x having perovskite structure with regard to nitrogen oxides in Mat. Res. Bull., 14, 133 (1979). Teraoka et al. reported decomposition activity of lanthanum-strontium-cobalt (manganese, iron, copper) with regard to NO.sub.x showed 30 to 75% of NO.sub.x conversion at high temperature of 700.degree. to 800.degree. C. (Chem. Lett., 1 (1990)). Shimada et al. reported that itrium-barium-copper oxide and itrium-barium-copper/magnesia showed 73% of decomposition activity at a temperature of over 600.degree. C. (Chem. Lett., 1 (1988)). However, Iwamoto and Hamada et al. also found that the activity of SrFeO.sub.3-x oxide, lanthanum-cobalt oxide and itrium-barium-copper oxide was reduced drastically in the presence of 5% oxygen, which resulted from low surface area of the those oxides (Catal. Today, 10, 57 (1991)). Examples of utilizing mixed oxide of perovskite type for the treatment of nitrogen oxides were disclosed in Japanese Patent 05 38 435. In this patent, as catalysts for removing air pollutants containing hydrocarbon, carbon monoxide and nitrogen oxides, mixed oxides including rare earth elements and oxides thereof, alumina, zirconia, iron oxides, bismuth oxides, complex perovskite containing more than one of noble metal were suggested, which were known to have heat resistance and catalyst stability.
Mixed metal oxides with perovskite structure have some drawbacks, for example, they show the decomposition activity for NO.sub.x only at high temperature, and their activity is drastically reduced in the presence of oxygen.
Therefore, the present inventors have made extensive studies to solve the problems relating to the mixed metal oxides with perovskite structure. As a result the present inventors have now found that the mixed metal oxides/zeolite catalyst produced by supporting mixed metal oxides on ZSM-5 zeolite support can be used in selective catalytic reduction of NO.sub.x using hydrocarbon as a reducing agent in oxygen excess and thus the activity of selective catalytic reduction of that catalyst can be maximized at low temperature less than 400.degree. C.